Method for fabricating a strained structure

ABSTRACT

A field effect transistor including a substrate which includes, a fin structure, the fin structure having a top surface. The field effect transistor further including an isolation in the substrate and a source/drain (S/D) recess cavity below the top surface of the substrate disposed between the fin structure and the isolation structure. The S/D recess cavity includes a lower portion, the lower portion further includes a first strained layer, a first dielectric film and a second dielectric film, wherein the first strained layer is disposed between the first dielectric film and the second dielectric film. The S/D recess cavity further includes an upper portion including a second strained layer overlying the first strained layer, wherein a ratio of a height of the upper portion to a height of the lower portion ranges from about 0.8 to about 1.2.

PRIORITY CLAIM

The present application is a continuation of U.S. application Ser. No. 12/775,006, filed May 6, 2010, the disclosure of which is incorporated herein by reference in its entirety.

RELATED APPLICATIONS

The present application is related to U.S. patent application Ser. No. 12/707,788, filed on Feb. 18, 2010, titled MEMORY POWER GATING CIRCUIT AND METHODS; Ser. No. 12/758,426, filed on Apr. 12, 2010, titled FINFETS AND METHODS FOR FORMING THE SAME; Ser. No. 12/731,325, filed on Mar. 25, 2010, titled ELECTRICAL FUSE AND RELATED APPLICATIONS; Ser. No. 12/724,556, filed on Mar. 16, 2010, titled ELECTRICAL ANTI-FUSE AND RELATED APPLICATIONS; Ser. No. 12/757,203, filed on Apr. 9, 2010, titled STI STRUCTURE AND METHOD OF FORMING BOTTOM VOID IN SAME; Ser. No. 12/797,839, filed on Jun. 10, 2010, titled FIN STRUCTURE FOR HIGH MOBILITY MULTIPLE-GATE TRANSISTOR; Ser. No. 12/831,842, filed on Jul. 7, 2010, titled METHOD FOR FORMING HIGH GERMANIUM CONCENTRATION SiGe STRESSOR; Ser. No. 12/761,686, filed on Apr. 16, 2010, titled FINFETS AND METHODS FOR FORMING THE SAME; Ser. No. 12/766,233, filed on Apr. 23, 2010, titled FIN FIELD EFFECT TRANSISTOR; Ser. No. 12/757,271, filed on Apr. 9, 2010, titled ACCUMULATION TYPE FINFET, CIRCUITS AND FABRICATION METHOD THEREOF; Ser. No. 12/694,846, filed on Jan. 27, 2010, titled INTEGRATED CIRCUITS AND METHODS FOR FORMING THE SAME; Ser. No. 12/638,958, filed on Dec. 14, 2009, titled METHOD OF CONTROLLING GATE THICKNESS IN FORMING FINFET DEVICES; Ser. No. 12/768,884, filed on Apr. 28, 2010, titled METHODS FOR DOPING FIN FIELD-EFFECT TRANSISTORS; Ser. No. 12/731,411, filed on Mar. 25, 2010, titled INTEGRATED CIRCUIT INCLUDING FINFETS AND METHODS FOR FORMING THE SAME; Ser. No. 12/775,006, filed on May 6, 2010, titled METHOD FOR FABRICATING A STRAINED STRUCTURE; Ser. No. 12/886,713, filed Sep. 21, 2010, titled METHOD OF FORMING INTEGRATED CIRCUITS; Ser. No. 12/941,509, filed Nov. 8, 2010, titled MECHANISMS FOR FORMING ULTRA SHALLOW JUNCTION; Ser. No. 12/900,626, filed Oct. 8, 2010, titled TRANSISTOR HAVING NOTCHED FIN STRUCTURE AND METHOD OF MAKING THE SAME; Ser. No. 12/903,712, filed Oct. 13, 2010, titled FINFET AND METHOD OF FABRICATING THE SAME; 61/412,846, filed Nov. 12, 2010, 61/394,418, filed Oct. 19, 2010, titled METHODS OF FORMING GATE DIELECTRIC MATERIAL and 61/405,858, filed Oct. 22, 2010, titled METHODS OF FORMING SEMICONDUCTOR DEVICES.

TECHNICAL FIELD

This disclosure relates to integrated circuit fabrication, and more particularly to a field effect transistor with a strained structure.

BACKGROUND

As the semiconductor industry has progressed into nanometer technology process nodes in pursuit of higher device density, higher performance, and lower costs, challenges from both fabrication and design issues have resulted in the development of three dimensional designs, such as a fin field effect transistor (FinFET). A typical FinFET is fabricated with a thin vertical “fin” (or fin structure) extending from a substrate, for example, etched into a silicon layer of the substrate. The channel of the FinFET is formed in this vertical fin. A gate is provided over (e.g., wrapping) the fin. Having a gate on both sides of the channel allows gate control of the channel from both sides. Further advantages of FinFET comprises reducing the short channel effect and enabling higher current flow.

FIG. 1A shows an isometric view of a conventional FinFET 100, and FIG. 1B illustrates a cross-sectional view of the FinFET 100 taken along the line a-a of FIG. 1A. The fin 104/108 comprises a raised active region 104 above a semiconductor substrate 102. Fin 104/108 is surrounded by a shallow trench isolation (STI) structure 106. A gate structure 110 comprising a gate dielectric 112, a gate electrode 114, and an optional hardmask layer 116 is formed above the fin 104/108. Sidewall spacers 118 are formed on both sides of the gate structure 110. Further, a portion of the fin 104/108 contains strained structures 108 in source and drain (S/D) recess cavities of the FinFET 100. The strained structures 108 are formed after a fin recessing process and an epitaxial growth step. The strained structures 108 utilizing epitaxial silicon germanium (SiGe) may be used to enhance carrier mobility.

However, there are challenges to implement such features and processes in complementary metal-oxide-semiconductor (CMOS) fabrication. As the gate length and spacing between devices decrease, these problems are exacerbated. For example, an ordered atomic arrangement does not exist due to lattice mismatch between the portion 104 of the fin 104/108 and strained portions 108. Thus, strain-induced crystal defects 108 a may become embedded in the strained structure 108. The crystal defects 108 a may provide carrier transportation paths during device operation, thereby increasing the likelihood of device instability and/or device failure.

Accordingly, what is needed is methods for fabricating a reduced-defect strained structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1A shows an isometric view of a conventional FinFET;

FIG. 1B illustrates a cross-sectional view of the FinFET taken along the line a-a of FIG. 1A;

FIG. 2 is a flowchart illustrating a method for fabricating strained structures according to various aspects of the present disclosure;

FIGS. 3A-F show schematic cross-sectional views of a FinFET comprising a strained structure at various stages of fabrication according to various aspects of the present disclosure; and

FIGS. 4A-E show schematic cross-sectional views of a FinFET comprising a strained structure at various stages of fabrication according to various aspects of the present disclosure.

DESCRIPTION

It is understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Referring to FIG. 2, illustrated is a flowchart of a method 200 for fabricating a semiconductor device according to various aspects of the present disclosure. The method 200 begins with block 202 in which a substrate is provided. The method 200 continues with block 204 in which a recess cavity comprising an upper portion and a lower portion may be formed in the substrate, wherein one sidewall of the recess cavity is dielectric and other sidewall of the recess cavity is the substrate. The method 200 continues with block 206 in which a dielectric film may be formed on the substrate sidewall portion and a bottom portion of the recess cavity. The method 200 continues with block 208 in which removing the dielectric film may include removing the dielectric film on the bottom portion of the recess cavity. The method 200 continues with block 210 in which epi-growing a first strained layer may be epi-grown in the lower portion of the recess cavity adjacent to a portion of the dielectric film. The method 200 continues with block 212 in which a portion of the dielectric film not adjacent to the first strained layer may be removed. The method 200 continues with block 214 in which a second strained layer may be epi-grown in the upper portion of the recess cavity. The discussion that follows illustrates various embodiments of semiconductor devices that can be fabricated according to the method 200 of FIG. 2.

Referring to FIGS. 3A-3F and 4A-4E, illustrated are schematic cross-sectional views of strained structures 308, 408 (in FIGS. 3F and 4E) of semiconductor devices 300, 400 at various stages of fabrication according to various aspects of the present disclosure. As employed in the present disclosure, the term semiconductor devices 300, 400 refer to a FinFET. The FinFET refers to any fin-based, multi-gate transistor. The semiconductor devices 300, 400 may be included in a microprocessor, memory cell, and/or other integrated circuit (IC). It is noted that the method of FIG. 2 does not produce completed semiconductor devices 300, 400. Completed semiconductor devices 300, 400 may be fabricated using complementary metal-oxide-semiconductor (CMOS) technology processing. Accordingly, it is understood that additional processes may be provided before, during, and after the method 200 of FIG. 2, and that some other processes may only be briefly described herein. Also, FIGS. 2 through 4E are simplified for a better understanding of the present disclosure. For example, although the figures illustrate the semiconductor devices 300, 400, it is understood the IC may comprise a number of other devices comprising resistors, capacitors, inductors, fuses, etc.

Referring to FIG. 3A, a substrate 102 is provided having a fin structure 104. In one embodiment, the substrate 102 comprises a crystalline silicon substrate (e.g., wafer). The substrate 102 may comprise various doped regions depending on design requirements (e.g., p-type substrate or n-type substrate). In some embodiments, the doped regions may be doped with p-type or n-type dopants. For example, the doped regions may be doped with p-type dopants, such as boron or BF₂; n-type dopants, such as phosphorus or arsenic; and/or combinations thereof. The doped regions may be configured for an n-type FinFET, or alternatively configured for a p-type FinFET.

The substrate 102 may alternatively be made of some other suitable elementary semiconductor, such as diamond or germanium; a suitable compound semiconductor, such as gallium arsenide, silicon carbide, indium arsenide, or indium phosphide; or a suitable alloy semiconductor, such as silicon germanium carbide, gallium arsenic phosphide, or gallium indium phosphide. Further, the substrate 102 may include an epitaxial layer (epi-layer), may be strained for performance enhancement, and/or may include a silicon-on-insulator (SOI) structure.

The fin structure 104, formed over the substrate 102, comprises one or more fins. In the present embodiment, for simplicity, the fin structure 104 comprises a single fin. The fin comprises any suitable material, for example, the fin structure 104 comprises silicon. The fin structure 104 may further comprise a capping layer disposed on the fin, which may be a silicon-capping layer.

The fin structure 104 is formed using any suitable process comprising various deposition, photolithography, and/or etching processes. An exemplary photolithography process may include forming a photoresist layer (resist) overlying the substrate 102 (e.g., on a silicon layer), exposing the resist to a pattern, performing a post-exposure bake process, and developing the resist to form a masking element including the resist. The masking element may then be used to etch the fin structure 104 into the silicon layer. The fin structure 104 may be etched using reactive ion etching (RIE) processes and/or other suitable processes. In an example, the silicon fin 104 is formed by using patterning and etching of a portion of the silicon substrate 102. In another example, silicon fins of the fin structure 104 may be formed by using patterning and etching of a silicon layer deposited overlying an insulator layer (for example, an upper silicon layer of a silicon-insulator-silicon stack of an SOI substrate).

Isolation structure 106 may be formed on the substrate 102 to isolate the various doped regions. The isolation structure 106 may utilize isolation technology, such as local oxidation of silicon (LOCOS) or shallow trench isolation (STI), to define and electrically isolate the various doped regions. In the present embodiment, the isolation structure 106 includes a STI. The isolation structure 106 may comprise silicon oxide, silicon nitride, silicon oxynitride, fluoride-doped silicate glass (FSG), a low-K dielectric material, and/or combinations thereof. The isolation structure 106, and in the present embodiment, the STI, may be formed by any suitable process. As one example, the formation of the STI may include patterning the semiconductor substrate 102 by a conventional photolithography process, etching a trench in the substrate 102 (for example, by using a dry etching, wet etching, and/or plasma etching process), and filling the trench (for example, by using a chemical vapor deposition process) with a dielectric material. In some embodiments, the filled trench may have a multi-layer structure such as a thermal oxide liner layer filled with silicon nitride or silicon oxide.

Still referring to FIG. 3A, a gate stack 110 is formed over the substrate 102 and over a portion of the fin structure 104. The gate stack 110 typically comprises a gate dielectric layer 112 and a gate electrode layer 114. The gate stack 110 may be formed using any suitable process, including the processes described herein.

In one example, the gate dielectric layer 112 and gate electrode layer 114 are sequentially deposited on the substrate 102 and over a portion of the fin structure 104. In some embodiments, the gate dielectric layer 112 may include silicon oxide, silicon nitride, silicon oxy-nitride, or high-k dielectric. High-k dielectrics comprise metal oxides. Examples of metal oxides used for high-k dielectrics include oxides of Li, Be, Mg, Ca, Sr, Sc, Y, Zr, Hf, Al, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and mixtures thereof. In the present embodiment, the gate dielectric layer 112 is a high-k dielectric layer with a thickness in the range of about 10 to 30 angstroms. The gate dielectric layer 112 may be formed using a suitable process such as atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), thermal oxidation, UV-ozone oxidation, or combinations thereof. The gate dielectric layer 112 may further comprise an interfacial layer (not shown) to reduce damage between the gate dielectric layer 112 and the fin structure 104. The interfacial layer may comprise silicon oxide.

In some embodiments, the gate electrode layer 114 may comprise a single layer or multilayer structure. In the present embodiment, the gate electrode layer 114 may comprise poly-silicon. Further, the gate electrode layer 114 may be doped poly-silicon with uniform or non-uniform doping. Alternatively, the gate electrode layer 114 may include a metal such as Al, Cu, W, Ti, Ta, TiN, TiAl, TiAlN, TaN, NiSi, CoSi, other conductive materials with a work function compatible with the substrate material, or combinations thereof. In the present embodiment, the gate electrode layer 114 comprises a thickness in the range of about 30 nm to about 60 nm. The gate electrode layer 114 may be formed using a suitable process such as ALD, CVD, PVD, plating, or combinations thereof.

Then, a layer of photoresist is formed over the gate stack 110 by a suitable process, such as spin-on coating, and patterned to form a patterned photoresist feature by a proper lithography patterning method. In one embodiment, a width of the patterned photoresist feature is in the range of about 15 to 45 nm. The patterned photoresist feature can then be transferred using a dry etching process to the underlying layers (i.e., the gate electrode layer 114 and the gate dielectric layer 112) to form the gate stack 110. The photoresist layer may be stripped thereafter.

In another example, a hard mask layer 116 is formed over the gate stack 110; a patterned photoresist layer is formed on the hard mask layer 116; the pattern of the photoresist layer is transferred to the hard mask layer 116 and then transferred to the gate electrode layer 114 and the gate dielectric layer 112 to form the gate stack 110. The hard mask layer 116 comprises silicon oxide. Alternatively, the hard mask layer 116 may optionally comprise silicon nitride, silicon oxynitride, and/or other suitable dielectric materials, and may be formed using a method such as CVD or PVD. The hard mask layer 116 has a thickness in the range from about 100 to 800 angstroms.

Still referring to FIG. 3A, the semiconductor device 300 further comprises a dielectric layer 118 formed over the substrate 102 and the gate stack 110. The dielectric layer 118 may include silicon oxide, silicon nitride, silicon oxy-nitride, or other suitable material. The dielectric layer 118 may comprise a single layer or multilayer structure. The dielectric layer 118 may be formed by CVD, PVD, ALD, or other suitable technique. The dielectric layer 118 comprises a thickness ranging from about 5 to 15 nm. Then, an anisotropic etching is performed on the dielectric layer 118 to form a pair of spacers 118 on two sides of the gate stack 110.

Still referring to FIG. 3A, other portions of the fin structure 104 (i.e., portions other than where the gate stack 110 and spacers 118 are formed thereover) are recessed to form source and drain (S/D) recess cavities 130 below a top surface of the substrate 102 disposed between the gate stack 110 and the isolation structure 106. In one embodiment, using the pair of spacers 118 as hard masks, a biased etching process is performed to recess a top surface of the fin structure 104 that are unprotected or exposed to form the S/D recess cavities 130. In an embodiment, the etching process may be performed under a pressure of about 1 mTorr to 1000 mTorr, a power of about 50 W to 1000 W, a bias voltage of about 20 V to 500 V, at a temperature of about 40° C. to 60° C., using a HBr and/or Cl2 as etch gases. Also, in the embodiments provided, the bias voltage used in the etching process may be tuned to allow better control of an etching direction to achieve desired profiles for the S/D recess cavities 130. The recess cavity 130 may comprise an upper portion 130 u and a lower portion 130 l separated by the dotted line in FIG. 3A. One sidewall 130 i of the recess cavity 130 is dielectric and other sidewall 130 f of the recess cavity 130 is the substrate 102. In one embodiment, a ratio of a height of the upper portion 130 u to a height of the lower portion 130 l may be from 0.8 to 1.2. In some embodiments, a height 130 a between the top surface of the substrate 102 and a bottom of the S/D recess cavity 130 is in the range of about 300 to 2000 nm.

Referring to FIG. 3B, following formation of the recess cavity 130, a dielectric film 132 may be formed along the substrate surface of the recess cavity 130. The dielectric film 132 comprises a sidewall portion 132 w and a bottom portion 132 b. The dielectric film 132 may be formed of silicon oxide or silicon oxynitride grown using a thermal oxidation process. For example, the dielectric film 132 can be grown by a rapid thermal oxidation (RTO) process or in a conventional annealing process, which includes oxygen or NO₂. A thickness t₁ of the dielectric film 132 may be in the range of about 20 to 100 angstroms.

Referring to FIG. 3C, subsequent to the formation of the dielectric film 132, a dry etching process is performed to remove the bottom portion 132 b of the dielectric film 132, whereby the sidewall portion 132 w of the dielectric film 132 is not removed. For example, the dry etching process may be a plasma etch process performed under a source power of about 120 to 160 W, and a pressure of about 450 to 550 mTorr, using BF3, H2, and Ar as etching gases.

Referring to FIG. 3D, after the bottom portion 132 b of the dielectric film 132 is removed, a first strained layer 136 is epi-grown in the lower portion 130 l of the recess cavities 130 adjacent to a portion of the dielectric film 132. In one embodiment, a first strained layer 136 comprising silicon germanium (SiGe) is epi-grown by a low-pressure chemical vapor deposition (LPCVD) process. The first strained layer 136 may serve as a relaxation layer and trap defects 136 a to eliminate crystal defects in a second strained layer 138 (shown in FIG. 3F) in the source and drain regions of the n-type FinFET. The LPCVD process is performed at a temperature of about 400 to 800° C. and under a pressure of about 1 to 200 Torr, using SiH₄ and GeH₄ as reaction gases. In another embodiment, a first strained layer 136 comprising silicon carbon (SiC) is epi-grown by a LPCVD process. The first strained layer 136 may serve as a relaxation layer and trap defects 136 a to eliminate crystal defects in a second strained layer 138 (shown in FIG. 3F) in the source and drain regions of the p-type FinFET. The LPCVD process is performed at a temperature of about 400 to 800° C. and under a pressure of about 1 to 200 Torr, using SiH₄ and CH₄ as reaction gases. A thickness t₂ of the first strained layer 136 may be in the range of about 15 to 45 nm. The thickness t₁ of the dielectric film 132 is less than the thickness t₂ of the first strained layer 136.

Referring to FIG. 3E, subsequent to the formation of the first strained layer 136, a top portion of the sidewall portion 132 w of the dielectric film 132 not adjacent to the first strained layer 136 has been removed using a wet etching process, for example, by dipping the substrate 102 in hydrofluoric acid (HF), exposing a top surface 132 a of the remaining sidewall portion 132 w of the dielectric film 132. Because the wet etching process has higher etch selectivity for oxide than to silicon, SiGe, and SiC, the etch process removes the dielectric film 132 faster than the fin structure 104 and the first strained layer 136.

In the present embodiment, the first strained layer 136 is disposed between the isolation structure 106 and the remaining sidewall portion 132 w of the dielectric film 132. In an embodiment, a top surface 136 b of the first strained layer 136 and the top surface 132 a of the remaining sidewall portion 132 w of the dielectric film 132 are substantially aligned. In another embodiment, the top surface 136 b of the first strained layer 136 and the top surface 132 a of the remaining sidewall portion 132 w of the dielectric film 132 are below a top surface 106 a of the isolation structure 106.

Referring to FIG. 3F, after the top portion of the sidewall portion 132 w of the dielectric film 132 is removed, a second strained layer 138 overlying the first strained layer 136 and remaining sidewall portion 132 w of the dielectric film 132 is epi-grown in the upper portion 130 u of the recess cavities 130 in the fin structure 104. Further, the first strained layer 136, remaining sidewall portion 132 w of the dielectric film 132, and second strained layer 138 are collectively hereinafter referred to as a strained structure 308. It should be noted that the first strained layer 136 serves as a relaxation layer and may trap defects 136 a to eliminate crystal defects in the second strained layer 138. Crystal defects in the second strained layer 138 may provide carrier transportation paths during device operation, thereby increasing the likelihood of device instability and/or device failure. Accordingly, the above method of fabricating a semiconductor device 300 may form a reduced-defect strained structure 308 to enhance carrier mobility and upgrade the device performance.

In one embodiment, the second strained layer 138, such as silicon carbide (SiC), is epi-grown by a LPCVD process to form the source and drain regions of the n-type FinFET. An example the LPCVD process for the growth of SiC is performed at a temperature of about 400 to 800° C. and under a pressure of about 1 to 200 Torr, using SiH₄ and CH₄ as reaction gases. In another embodiment, the second strained layer 138, such as silicon germanium (SiGe), is epi-grown by a LPCVD process to form the source and drain regions of the p-type FinFET. The LPCVD process is performed at a temperature of about 400 to 800° C. and under a pressure of about 1 to 200 Torr, using SiH₄ and GeH₄ as reaction gases. In still another embodiment, the second strained layer 138, such as silicon, is epi-grown by a LPCVD process to form the source and drain regions of both the p-type FinFET and n-type FinFET. The LPCVD process is performed at a temperature of about 400 to 800° C. and under a pressure of about 1 to 200 Torr, using SiH₄ as a reaction gas.

Alternatively, FIG. 4A shows the substrate 102 of FIG. 3A after deposition of a dielectric film 142 by a CVD process. The dielectric film 142 formed by CVD will deposit over all exposed surfaces, and thus may be formed on the isolation structure 106, hard mask layer 116, spacers 118, and recess cavities 130. The dielectric film 142 may comprise a first sidewall portion 142 w, a second sidewall portion 142 s, and a bottom portion 142 b. The dielectric film 142 may be formed of silicon oxide or silicon oxynitride deposited using a CVD process. For example, the dielectric film 142 can be deposited under a pressure less than 10 mTorr and a temperature of about 350° C. to 500° C., using SiH₄ and N₂O as reacting precursors. A thickness t₃ of the dielectric film 142 may be in the range of about 20 to 100 angstroms.

Referring to FIG. 4B, subsequent to the formation of the dielectric film 142, a dry etching process is performed to remove the bottom portion 142 b of the dielectric film 142, whereby the first sidewall portion 142 w and second sidewall portion 142 s of the dielectric film 142 are not removed. For example, the dry etching process may be performed under a source power of about 120 to 160 W, and a pressure of about 450 to 550 mTorr, using BF3, H2, and Ar as etching gases.

Referring to FIG. 4C, after the bottom portion 142 b of the dielectric film 142 removing process, a first strained layer 146 is epi-grown in the lower portion 130 l of the recess cavities 130 adjacent to a portion of the dielectric film 142. In one embodiment, a first strained layer 146 comprising silicon germanium (SiGe) is epi-grown by a LPCVD process. The first strained layer 146 may serve as a relaxation layer and trap defects 146 a to eliminate crystal defects in a second strained layer 148 (shown in FIG. 4E) in the source and drain regions of the n-type FinFET. The LPCVD process is performed at a temperature of about 400 to 800° C. and under a pressure of about 1 to 200 Torr, using SiH₄ and GeH₄ as reaction gases. In another embodiment, a first strained layer 146 comprising silicon carbide (SiC) is epi-grown by a LPCVD process. The first strained layer 146 may serve as a relaxation layer and trap defects 146 a to eliminate crystal defects in the second strained layer 148 (shown in FIG. 4E) in the source and drain regions of the p-type FinFET. In one embodiment, LPCVD process for SiC deposition is performed at a temperature of about 400 to 800° C. and under a pressure of about 1 to 200 Torr, using SiH₄ and CH₄ as reaction gases. A thickness t₄ of the first strained layer 146 may be in the range of about 12 to 40 nm. The thickness t₃ of the dielectric film 142 is less than the thickness t₄ of the first strained layer 146.

Referring to FIG. 4D, subsequent to the formation of the first strained layer 146, top portions of the first and second sidewall portions 142 w, 142 s of the dielectric film 142 not adjacent to the first strained layer 146 are removed using a wet etching process, for example, by dipping the substrate 102 in hydrofluoric acid (HF), exposing top surfaces 142 a, 142 b of the remaining first and second sidewall portions 142 w, 142 s of the dielectric film 142. Because the wet etching process preferentially etches oxide over silicon, SiGe, and SiC, the etch process removes the dielectric film 142 faster than the fin structure 104 and the first strained layer 146.

In the present embodiment, the first strained layer 146 is disposed between the isolation structure 106 and the remaining first sidewall portion 142 w of the dielectric film 142. Further, the remaining second sidewall portion 142 s of dielectric film 142 is between the first strained layer 146 and the isolation structure 106. In an embodiment, a top surface 146 b of the first strained layer 146 and the top surfaces 142 a, 142 b of the remaining first and second sidewall portions 142 w, 142 s of the dielectric film 142 are substantially aligned. In another embodiment, the top surface 146 b of the first strained layer 136 and the top surfaces 142 a, 142 b of the remaining first and second sidewall portions 142 w, 142 s of the dielectric film 142 are below the top surface 106 a of the isolation structure 106.

Referring to FIG. 4E, after the top portions of the first and second sidewall portions 142 w, 142 s of the dielectric film 142 are removed, a second strained layer 148 overlying the first strained layer 146 and remaining first and second sidewall portions 142 w, 142 s of the dielectric film 142 is epi-grown in the upper portion 130 u of the recess cavities 130. Further, the first strained layer 146, remaining first sidewall portion 142 w and second sidewall portion 142 w of the dielectric film 142, and second strained layer 148 are collectively hereinafter referred to as a strained structure 408. It should be noted that the first strained layer 146 serves as a relaxation layer and may trap defects 146 a to eliminate crystal defects in the second strained layer 148. Crystal defects in the second strained layer 148 may provide carrier transportation paths during device operation, thereby increasing the likelihood of device instability and/or device failure. Accordingly, the above method of fabricating a semiconductor device 400 may form a reduced-defect strained structure 408 to enhance carrier mobility and upgrade the device performance.

In one embodiment a second strained layer 148 comprising silicon carbide (SiC) is epi-grown by a LPCVD process to form the source and drain regions of the n-type FinFET. The LPCVD process is performed at a temperature of about 400 to 800° C. and under a pressure of about 1 to 200 Torr, using SiH₄ and CH₄ as reaction gases. In another embodiment a second strained layer 148 comprising silicon germanium (SiGe) is epi-grown by a LPCVD process to form the source and drain regions of the p-type FinFET. The LPCVD process is performed at a temperature of about 400 to 800° C. and under a pressure of about 1 to 200 Torr, using SiH₄ and GeH₄ as reaction gases. In still another embodiment a second strained layer 148 comprising silicon is epi-grown by a LPCVD process to form the source and drain regions of both the p-type FinFET and n-type FinFET. The LPCVD process is performed at a temperature of about 400 to 800° C. and under a pressure of about 1 to 200 Torr, using SiH₄ as a reaction gas.

After the steps shown in FIGS. 2, 3 and 4 have been performed, subsequent processes, comprising silicidation and interconnect processing, are typically performed to complete the semiconductor device 300 and 400 fabrication.

One aspect of this description relates to a field effect transistor. The field effect transistor includes a substrate including a fin structure, the fin structure having a top surface. The field effect transistor further including an isolation in the substrate and a source/drain (S/D) recess cavity below the top surface of the substrate disposed between the fin structure and the isolation structure. The S/D recess cavity includes a lower portion, the lower portion further includes a first strained layer, a first dielectric film and a second dielectric film, wherein the first strained layer is disposed between the first dielectric film and the second dielectric film. The S/D recess cavity further includes an upper portion including a second strained layer overlying the first strained layer, wherein a ratio of a height of the upper portion to a height of the lower portion ranges from about 0.8 to about 1.2.

Another aspect of this description relates to a method for fabricating a semiconductor device. The method includes forming a recess cavity comprising an upper portion and a lower portion in a substrate, wherein the recess cavity includes a sidewall defined by an isolation structure. The method further includes forming a dielectric film on a bottom portion and sidewalls of the recess cavity and removing the dielectric film on the bottom portion of the recess cavity. The method further includes forming a first strained layer in the lower portion of the recess cavity between the dielectric film on the sidewalls of the recess cavity and forming a second strained layer over the first strained layer in the upper portion of the recess cavity.

Still another aspect of this description relates to a method of fabricating a field effect transistor. The method includes forming a plurality of source/drain (S/D) recess cavities below a top surface of a substrate, the substrate includes a fin structure and a plurality of isolation structures, each S/D recess cavity of the plurality of S/D recess cavities disposed between the fin structure and a corresponding isolation structure of the plurality of isolation structures. The method further includes forming a lower portion in each S/D recess cavity of the plurality of recess cavities. Forming the lower portion includes forming a first strained layer between a first dielectric film and a second dielectric film. The method further includes forming a second strained layer overlying the first strained layer, wherein a ratio of a height of the second strained layer to a height of the first strained layer ranges from about 0.8 to about 1.2.

While the preferred embodiments have been described by way of example, it is to be understood that the disclosure is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the disclosure should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. The disclosure can be used to form or fabricate a strained structure for a semiconductor device. In this way, a strained structure having no defect in a semiconductor device is fabricated. 

What is claimed is:
 1. A field effect transistor comprising: a substrate comprising a fin structure, the fin structure having a top surface; an isolation in the substrate; and a source/drain (S/D) recess cavity below the top surface of the substrate disposed between the fin structure and the isolation structure, the S/D recess cavity comprising: a lower portion, the lower portion further comprising a first strained layer, a first dielectric film and a second dielectric film, wherein the first strained layer is disposed between the first dielectric film and the second dielectric film; and an upper portion comprising a second strained layer overlying the first strained layer, wherein a ratio of a height of the upper portion to a height of the lower portion ranges from about 0.8 to about 1.2.
 2. The field effect transistor of claim 1, wherein the first dielectric film and the second dielectric film comprise a same material.
 3. The field effect transistor of claim 1, wherein the first strained layer comprises trapped defects.
 4. The field effect transistor of claim 1, wherein a thickness of the first strained layer in a direction parallel to a top surface of the substrate ranges from about 12 nanometers (nm) to about 40 nm.
 5. The field effect transistor of claim 1, wherein a thickness of the first dielectric layer in a direction parallel to a top surface of the substrate ranges from about 20 angstroms (Å) to about 100 Å.
 6. The field effect transistor of claim 5, wherein the thickness of the first dielectric layer is less than a thickness of the first strained layer in a direction parallel to a top surface of the substrate.
 7. The field effect transistor of claim 5, wherein the thickness of the first dielectric layer is equal to a thickness of the second dielectric layer in a direction parallel to a top surface of the substrate.
 8. The field effect transistor of claim 1, wherein a top surface of the first dielectric layer is substantially aligned with a top surface of the first strained layer and a top surface of the second dielectric layer.
 9. A method for fabricating a semiconductor device, comprising: forming a recess cavity comprising an upper portion and a lower portion in a substrate, wherein the recess cavity includes a sidewall defined by an isolation structure; forming a dielectric film on a bottom portion and sidewalls of the recess cavity; removing the dielectric film on the bottom portion of the recess cavity; forming a first strained layer in the lower portion of the recess cavity between the dielectric film on the sidewalls of the recess cavity, wherein the first strained layer comprises at least one trapped defect; and forming a second strained layer over the first strained layer in the upper portion of the recess cavity.
 10. The method of claim 9, wherein removing the dielectric film on the bottom portion of the recess cavity comprises: dry etching the dielectric film at a pressure ranging from about 450 milliTorr (mTorr) to about 550 mTorr, using an etching gas comprising boron trifluoride (BF₃) and hydrogen gas, (H₂).
 11. The method of claim 9, wherein forming the first strained layer comprises: epitaxially growing the first strained material at a temperature of about 400° C. to about 800° C. under a pressure of about 1 Torr to about 200 Torr, using silane (SiH₄) and methane (CH₄) as reaction gases.
 12. The method of claim 9, further comprising removing the dielectric layer from the sidewalls of the recess cavity above the first strained layer.
 13. The method of claim 12, wherein removing the dielectric layer from the sidewalls of the recess cavity above the first strained layer comprises: dipping the semiconductor device in hydrofluoric acid (HF).
 14. The method of claim 9, wherein forming the second strained layer comprises: epitaxially growing the second strained material at a temperature of about 400° C. to about 800° C. under a pressure of about 1 Torr to about 200 Torr, using silane (SiH₄) as a reaction gas.
 15. The method of claim 14, wherein forming the second strained layer further comprises using methane (CH₄) as a second reaction gas, and the second strained layer comprises silicon carbide (SiC).
 16. The method of claim 14, wherein forming the second strained layer further comprises using germane (GeH₄) as a second reaction gas, and the second strained layer comprises silicon germanium (SiGe).
 17. A method of fabricating a field effect transistor, the method comprising: forming a plurality of source/drain (S/D) recess cavities below a top surface of a substrate, the substrate comprising a fin structure and a plurality of isolation structures, each S/D recess cavity of the plurality of S/D recess cavities disposed between the fin structure and a corresponding isolation structure of the plurality of isolation structures; forming a lower portion in each S/D recess cavity of the plurality of recess cavities, forming the lower portion comprising: forming a first strained layer between a first dielectric film and a second dielectric film; and forming a second strained layer overlying the first strained layer, wherein a ratio of a height of the second strained layer to a height of the first strained layer ranges from about 0.8 to about 1.2.
 18. The method of claim 17, wherein forming the lower portion further comprises: forming a continuous dielectric layer along sidewalls and a bottom portion of the S/D recess cavity; and removing the continuous dielectric layer on the bottom portion of the S/D recess cavity to form the first dielectric layer and second dielectric layer.
 19. The method of claim 17, wherein forming the lower portion further comprises: aligning a top surface of the first dielectric layer with a top surface of the first strained layer and a top surface of the second dielectric layer.
 20. The method of claim 17, further comprising forming a gate structure on top of the fin structure, wherein the fin structure is disposed between two S/D recess cavities of the plurality of S/D recess cavities. 